Without yeasts, unicellular fungi that reproduce by budding one cell on top of each other, we would not have leavened bread or the deliciousness known as beer. Yeasts work their magic on bread and beer by feasting on the sugar in flour or grains, and then releasing small amounts of carbon dioxide and ethanol alcohol. "Leavened bread was probably discovered by accident," Stephenson says. "When someone left some dough out and it came into contact with yeasts and began to rise."

The discovery of beer and wine probably came about in similar fashion, he says. Such libations may have contributed to the development of modern agriculture (if hunter-gathers decided to stay in one place to grow grains for Stone Age beer production), and important trade routes. "There's no question these things changed human history," he says.